A technique by which a transistor is formed using a semiconductor thin film formed over a substrate having an insulating surface has been attracting attention. The transistor has been applied to a wide range of semiconductor electronic devices such as an integrated circuit (IC) and an image display device (also simply referred to as a display device). While a silicon-based semiconductor material is widely known as a material for the semiconductor thin film applicable to the transistor, an oxide semiconductor has been attracting attention as another material.
For example, a technique is disclosed by which a transistor is formed using zinc oxide or an In—Ga—Zn-based oxide as an oxide semiconductor (see Patent Documents 1 and 2).
Meanwhile, it has been pointed out that if hydrogen is contained in an oxide semiconductor, a donor is generated at a level close to the conduction band (a shallow level) to lower the resistance (make the oxide semiconductor an n-type oxide semiconductor). Therefore, some measures need to be taken to prevent hydrogen from entering the oxide semiconductor at the time of forming the oxide semiconductor. Further, the following technique is also disclosed: a change of the threshold voltage is suppressed by reducing the amount of hydrogen in not only the oxide semiconductor but also a gate insulating film in contact with the oxide semiconductor (see Patent Document 3).